Female internal reproductive organs are composed of ovaries, uterus and fallopian tubes. The main function of the ovary is to produce eggs and synthesize ovarian hormones, while the uterus and fallopian tubes are reproductive organs. There are hundreds of thousands of follicles in the ovary, and each follicle contains 1 egg.
Prepubertal follicles basically have no function. In adolescence, under the action of gonadotropin in the anterior pituitary gland, immature follicles gradually develop and estrogen is synthesized. When the follicle matures and ovulates, the follicle wall collapses and the cells become larger and yellow, which is called corpus luteum. It synthesizes estrogen and produces progesterone at the same time.
With the change of ovary, the endometrium also changes periodically. Estrogen makes endometrium thicken, endometrial cells increase and enlarge, and interstitial arterioles become more tortuous and spiral, which is called proliferative endometrium. After ovulation, due to the interaction of estrogen and progesterone, endometrial edema occurs, and glands produce a lot of mucus and glycogen, and the thickness of endometrium increases from 1 mm to 6 mm, which is called secretory endometrium.
If the ovum discharged at this time is fertilized, the fertilized egg is transported to the uterus through the fallopian tube for development, which is called pregnancy. The pregnant tissue synthesizes chorionic gonadotropin to support the continuous development of ovarian corpus luteum. If the egg is not fertilized, the corpus luteum will shrink about 14 days after ovulation and stop secreting estrogen and progesterone. At this time, the blood vessels in the endometrium contract, and the endometrium is necrotic and falls off, causing bleeding and forming menstruation.
Therefore, the length of menstrual cycle depends on the length of ovarian cycle, which is generally 2 1-30 days, but it also varies from person to person, with 23-45 days, even three months or half a year 1 cycle. As long as it is regular, it generally belongs to normal menstruation.